This invention relates to emulsions useful in effecting antimicrobial activity against various micro-organisms and, more particularly, to novel oil-in-water emulsions containing lower alkyl esters of p-hydroxy-benzoic acid and which are freeze-thaw and heat stable in practical use.
Heretofore, it has been known that certain p-hydroxybenzoic acid lower alkyl esters such as iso-propyl p-hydroxybenzoate, isobutyl p-hydroxybenzoate and n-butyl p-hydroxybenzoate are useful as preservatives. Ueno U.S. Pat. No. 3,097,131 discloses that eutectic mixtures of p-hydroxybenzoic acid alkyl esters may be emulsified in an aqueous solution containing such colloid protecting substances as gelatin, carboxymethyl cellulose and the like. However, it is not believed that such emulsions have the desired degree of heat stability or freeze stability for all applications and also may be subject to "breaking" or separating into two phases. Swift U.S. Pat. No. 3,085,889 discloses the use of sorbitol to impart freeze-thaw stability to asphalt emulsions, resin emulsions and rubber emulsions, but does not suggest that heat stability for emulsions can thereby be achieved. In the copending, coassigned application Ser. No. 199,719, now U.S. Pat. No. 4,309,564. of Donald F. Loncrini and John J., Taylor entitled Methods for Preparing Eutectic Mixtures of p-Hydroxybenzoic Acid Esters, incorporated herein by reference, there are described methods for preparing eutectic oil mixtures of three or more lower alkyl esters of p-hydroxybenzoic acid such as isopropyl p-hydroxybenzoate, isobutyl p-hydroxybenzoate and n-butyl p-hydroxybenzoate. It would be advantageous to provide means for formulating such eutectic oil mixtures into oil-in-water emulsions which exhibit freeze-thaw and heat stability for use as antimicrobial agents or compositions.